This invention relates to an antenna coupler interposed between an antenna and communication systems which both use it for receiving and transmitting signals in different frequency bands.
When two communication systems operate in frequency bands which are relatively close to each other, it has been known to connect them to a common antenna. In FIG. 4, which shows a prior art structure for providing a common antenna 20 to communication systems each operating in a different frequency band, numeral 21 indicates a switch means comprising a coaxial switch or a PIN diode, numeral 22 indicates an antenna coupler for an 800 MHz frequency band, and numeral 23 indicates an antenna coupler for a 1.5 GHz frequency band. The reception terminals (Rx terminals) of the antenna couplers 22 and 23 are connected respectively to reception circuits for the 800 MHz and 1.5 GHz frequency bands, and the transmission terminals (Tx terminals) of the antenna couplers 22 and 23 are connected respectively to transmission circuits for the 800 MHz and 1.5 GHz frequency bands. The antenna 20 is selectively connected to either of the antenna couplers 22 and 23 through the switch means 21 for operations in the 800 MHz or 1.5 GHz band.
With a prior art antenna coupler thus formed, however, the number of constituent parts increases because of the need for the switch means. The cost of the coupler increases, if a coaxial switch is used as the switch means. If a PIN diode is substituted as the switch means, on the other hand, there are adverse effects such as deterioration in the insertion loss of the antenna couplers 22 and 23.